Infrared photometry and calcium triplet spectroscopy of the most metal-poor in situ globular cluster VVV-CL001

Context . The characterization of globular clusters (GCs) in the Galactic bulge is a challenging task due to high extinction and severe stellar crowding. VVV-CL001 is a poorly studied GC located in the inner bulge, known for its extremely old age, extreme velocity, and low metallicity. Given its unique properties, a detailed study of this cluster can provide valuable insights into the early chemical and dynamical evolution of the Milky Way (MW). Aims . The aim of this study was to derive the fundamental parameters of VVV-CL001 including metallicity, heliocentric radial velocity (RV), proper motions (PMs), structural properties, orbit, and age, in order to improve our understanding of its origin and role in the early evolution of the MW. Methods . We combined spectroscopic, astrometric, and photometric data to characterize VVV-CL001. Metallicity and RV were determined from medium-resolution spectra obtained with FORS2 at the Very Large Telescope. PMs were derived using Gaia DR3 data. Near-infrared photometry from the FourStar instrument on Magellan was used to refine the cluster’s position, construct a radial density profile, and estimate its age, distance, and reddening. Results . Our results confirm that VVV-CL001 is an old 12.1 −1.2 +1.0 Gyr), metal-poor ([Fe/H] = −2.25 ± 0.05 dex) globular cluster located at a heliocentric position of d ⊙ = 7.1 −1.1 +1.3 , with a reddening of E( J − K s ) = 1.40 −0.02 +0.01 . Its mean PMs are μ α * = −3.68 ± 0.09 mas yr −1 and µ δ = −1.76 ± 0.10 mas yr −1 , and it exhibits a RV of −334 ± 4 km s −1 . The cluster follows a retrograde-prograde eccentric ( e = 0.76 −0.14 +0.10 ) orbit, confined within the Galactic plane (|Z| max = 1.0 −0.32 +0.45 kpc) and inside the bar’s radius of influence ( R < 5 kpc), with a pericenter of r peri = 0.6 −0.2 +0.3 kpc and an apocenter of r apo = 4.5 −1.2 +2.5 kpc. Conclusions . These orbital properties, combined with its ancient age and low metallicity, strongly support an in situ origin for VVV-CL001 and likely membership of the disk GC system that was captured by the potential of the bar during its formation. Thus, VVV-CL001 emerges as a fossil remnant of the earliest phases of Galactic assembly and a valuable tracer of the population that contributed to the formation of the inner thick disk and bulge, which are likely part of the main progenitor of the MW. Our study highlights the relevance of detailed chemo-dynamical analyses in unveiling the origin of GCs in the inner Galaxy.